Fix CPU spin on half-closed WebSocket / silent body truncation

[ademar]

readPostData aborts silently on EOF or cancellation, leaving the caller with a zero-filled buffer and no indication that the read was short.

In WebSocket.readBytes / readBytesIntoByteSegment this caused the frame parser to see opcode=0 length=0 (a zero-length Continuation frame), return immediately, and loop forever - pegging a CPU core after the peer half-closed the connection.

In ConnectionFacade.getRawPostData the same bug silently truncated HTTP/1 request bodies, returning Ok with a partially-zeroed buffer instead of surfacing the broken connection.

The HTTP/2 reader (Http2.readBytes) already guards against this with an 'if offset = n' check; apply the same fix to the other three call sites.

Added a regression test in src/Suave.Tests/WebSocket.fs that opens a raw TCP socket, performs a valid WebSocket upgrade handshake, half-closes the client send side, and asserts the handler exits within a deadline.

[ademar]

@copilot Can you produce a test that opens a TCP socket, sends a valid WebSocket upgrade, then half-closes — and asserts the handler exits within a deadline.

[Copilot]

@copilot Can you produce a test that opens a TCP socket, sends a valid WebSocket upgrade, then half-closes — and asserts the handler exits within a deadline.

Added in 3ee47f6. There is now a regression test in /src/Suave.Tests/WebSocket.fs that opens a raw TcpClient, sends a valid WebSocket upgrade request, half-closes the socket (SocketShutdown.Send), and asserts the handler exits within 2 seconds.